Artificial kidney systems include a dialyzer and a dialysis machine which controls the operation of the dialyzer. The dialyzer is used to treat a patient's blood so as to remove water and waste products therefrom. The dialyzer includes a semipermeable membrane which separates blood and dialysis solution flowing through the dialyzer. Waste product removal occurs by mass transfer through the membrane and water removal occurs by ultrafiltration through the membrane.
In some dialysis machines the dialysis solution is drawn through the dialyzer under a negative pressure (i.e., below atmospheric pressure). Such machines normally include a negative pressure pump positioned downstream of the dialyzer for drawing the dialysis solution through the dialyzer and an adjustable negative pressure valve positioned upstream of the dialyzer for controlling the negative pressure in the dialyzer. One such machine is disclosed in U.S. Pat. No. 3,878,095 Frasier et al. A commercial machine embodying such a system is manufactured and sold by Baxter Travenol Laboratories and is identified as Proportioning Dialyzing Fluid Delivery System (5M 1352-5M 1355).
In the single-pass or flow-through machines, dialysis solution is either continuously prepared or is prepared in batches. The fresh dialysis solution is drawn through the dialyzer, through the machine and is discharged to a drain. There is also a second type of negative pressure system, known as the closed-loop system, in which a fixed volume of dialysis solution continuously recirculates through the dialyzer, and thus it is necessary to remove waste products from the solution by appropriate cleansing or regenerating apparatus.
The removal of water from the blood by the process of ultrafiltration relies on the pressure differential across the semipermeable membrane (i.e., the difference in pressure of the blood flowing under positive pressure through the dialyzer and the dialysis solution flowing under a negative pressure through the dialyzer). This pressure differential is commonly known as the transmembrane pressure and the amount of water removed from the blood is directly related thereto.
It is known to be desirable to control the amount of and the rate at which water is removed from the patient, since the removal of too much water at too fast a rate can result in one type of a side effect and water removal at too slow a rate can result in a different type of a side effect. In some existing systems, the ultrafiltration rate is related only to negative pressure. This relation is less accurate than relating the ultrafiltration to transmembrane pressure, since no provision is made for changes in blood pressure which affect the transmembrane pressure and thus the ultrafiltration rate.
U.S. Pat. Nos. 3,844,940; 3,979,284; and 3,990,973 appear to be representative of existing ultrafiltration systems. These patents disclose, for example: closed-circuit systems which require regeneration; systems relating ultrafiltration only to negative pressure; and an electrical measuring system having related transmembrane pressure controls.
However, the existing systems have not provided a desirable structure for use in a flow-through type machine as described above for determining, setting and controlling the rate of ultrafiltration.
It is therefore an object of this invention to provide an ultrafiltration system which permits accurate and controllable determination and setting of the ultrafiltration rate.
It is another object of this invention to also provide a system for controlling and relating ultrafiltration to transmembrane pressure.
These and other objects and advantages of this invention will become apparent from the following description and appended claims.